Nowadays the use of laundry washing machines, both “simple” laundry washing machines (i.e. laundry washing machines which can only wash and rinse laundry) and laundry washing-drying machines (i.e. laundry washing machines which can also dry laundry), is widespread.
In the present description the term “laundry washing machine” will refer to both simple laundry washing machines and laundry washing-drying machines.
Laundry washing machines generally comprise an external casing provided with a washing tub which contains a rotatable perforated drum where the laundry is placed.
A loading/unloading door ensures access to the drum.
Laundry washing machines typically comprise a water inlet circuit and a products supply unit for the introduction of water and washing/rinsing products (i.e. detergent, softener, etc.) into the tub.
Known laundry washing machines are also provided with drain liquid devices that may operate during different phases of the washing cycle in order to drain liquid from the bottom of the tub and to convey the liquid either to the outside or inside the same washing tub. In the latter case, the drain liquid device is part of a recirculation circuit.
The drain liquid devices typically drain water from the tub by means of a discharge pipe connected to the bottom of the tub.
The bottom of the washing tub typically receives a heater, usually an electrical resistance, which heats up the liquid wherein it is immersed.
The laundry washing machines of know type are also typically equipped with a level sensing device for sensing the liquid level in the machine. The liquid level detected by this device is opportunely used to correctly activate the water inlet circuit and the drain liquid device during the phases of the washing cycle.
Level sensing devices of known type comprise a pressure sensor, or pressure switch, which communicates with an air volume enclosed in an air chamber via a narrow conduit. The air chamber is typically arranged at the bottom part of the washing machine and is typically connected to the drain liquid device or directly to the bottom of the washing tub. The air chamber is connected to the drain liquid device or directly to the bottom of the washing tub in such a way that the liquid in the chamber reflects the liquid level in the machine. The air volume enclosed above the liquid in the chamber is therefore subjected to a pressure which is sensed by the pressure sensor and then translated into a measure of the liquid level in the machine.
A laundry washing machine showing an air chamber for a pressure sensor is disclosed in document WO2008107022A1.
The air chamber disclosed in such document comprises two portion: an upper portion communicating with the pressure sensor through a narrow conduit and a lower portion connected to an outlet of the washing tub.
The outlet of the washing tub comprises a cylindrically shaped collar at which a corresponding cylindrically shaped portion of the lower portion of the air chamber is inserted.
Therefore, between the washing tub and the air chamber a cylindrical liquid flowing path is defined wherein the liquid may flow.
When the liquid rises in the tub, a corresponding liquid column is also created in the air chamber. The liquid column applies a pressure to the air volume enclosed thereabove in the air chamber and inside the conduit. Said pressure is then sensed by the pressure sensor connected to the conduit.
As known, the pressure sensor starts to detect a pressure only when the liquid flowing path is completely obstructed by the liquid which flows from the washing tub towards the air chamber. If the liquid flowing path is not completely obstructed by the liquid, in fact, the air chamber pressure does not increase and remains at its normal pressure, i.e. at atmospheric pressure.
The pressure sensor, therefore, starts to detect a pressure only when the liquid inside the washing tub reaches a level at which the liquid flowing path is completely obstructed. This happens when the liquid inside the washing tub reaches a level corresponding to the upper part of the liquid flowing path, i.e. the upper part of the cylindrical liquid flowing path.
The above described known level sensing device, however, poses some drawbacks.
A first drawback posed by this known technique is constituted by the fact that the starting point of the detection for the level sensing device is not accurate. In fact, the starting point of detection occurs when the liquid inside the washing tub reaches a level corresponding to the upper part of the cylindrical liquid flowing path, which is practically constituted by the upper point of a circumference.
Therefore, even slight movements of the liquid around such starting detection level may cause intermittent detections of the liquid level by the pressure sensor.
This may determine a not stable level detection by the pressure sensor and a consequent wrong management of the washing cycle which is based on said liquid level.
Furthermore, a wrong level detection of low liquid level inside the washing tub may cause an undesired period of time during which the electric resistance is not immersed in the liquid which, in turn, may cause the burning of the electrical resistance.
The aim of the present invention is therefore solving the noted drawbacks and thus providing a laundry washing machine having an improved liquid level sensing device arrangement.
It is a first object of the invention to implement a laundry washing machine that makes it possible to increase the liquid level detection accuracy compared to machines of known type.
It is a further object of the invention to implement a laundry washing machine that makes it possible to increase reliability of the machine itself.
Another object of the present invention is to provide a laundry washing machine that lasts longer than machines of known type.
Advantages, objects, and features of the invention will be set forth in part in the description and drawings which follow and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention.